In conventional dual pressure air separation processes high oxygen purity is obtained by supplying a maximum amount of reboil to the argon stripping section of the low pressure column, and the argon recovery is limited by the amount of reboil and reflux availabnle to the argon rectification section of the low pressure column. This is illustrated e.g. in U.S. Pat. No. 2,934,908. In high efficiency flowsheets these limitations are usually even more severe, since in order to decrease the pressure of the high pressure rectifier, (and hence supply air pressure), some of the available reboil normally bypasses the argon stripper. The product oxygen withdrawal or delivery pressure is also usually decreased due to the low HP rectifier pressure.
U.S. Pat. Nos. 3,277,655, 3,327,489, 4,372,765, 4,410,343, and 4,254,629 all disclose low energy flowsheets involving lower than normal HP rectifier pressures, and all result in limited purity oxygen (below about 98%) due to reduced reboil available in the argon stripping section of the LP column. The first four reflect a dual pressure (two column) arrangement, whereas the latter reflects alternatively a triple pressure arrangement with split air supply pressure or a quadruple pressure column arrangement with single supply pressure.
U.S. Pat. No. 2,699,046 to Etienne reflects numerous triple pressure and one quadruple pressure column arrangements. Several of those arrangements also accomplish lower energy requirement at the expense of lower oxygen purity. One, FIG. 6, does not decrease separation energy but increases the purity of the nitrogen product.
U.S. Pat. No. 3,688,513 partly avoids the oxygen purity limitation of low energy triple pressure column flowsheets by incorporating an argon stripper at the bottom of the medium pressure column in addition to the one at the bottom of the LP column. The argon stripping duty is divided between the two strippers, and thus much of the reboil diverted from the LP column to the MP column is still effective in stripping argon. This configuration also incorporates pumped liquid recycle from the LP column overhead back to the MP column, in order to remove argon from the LP column.
This configuration has the disadvantage of not recovering byproduct argon, which in turn causes several additional disadvantages. Since LP column overhead liquid containing argon is recycled to the MP column, and the argon must eventually leave in the MP column, overhead gas, this causes a buildup of argon concentration levels throughout the MP column. The argon concentration must increase until the overhead nitrogen contains on the order of 1% argon, and hence the overhead liquid contains almost 3% (due to the relative volatility between N.sub.2 and Ar). The end result is much higher argon concentration in the oxygen rich liquid near the bottom of the MP column, requiring more trays and more reboil in the argon stripping sections of both the LP and MP columns. This equates to greater column pressure drops and hence higher required air supply pressure and higher compression energy (compared to a flowsheet in which the recycle and resulting buildup of argon is not required).
Once the adverse consequences of recycling argon so as to remove it with the nitrogen are discovered, the question occurs as to why the prior art disclosure so definitively emphasizes that recycle, and makes no mention whatever of e.g. argon withdrawl. Although the reason for this is not known with certainty, the discoveries reported in the following disclosure make apparent a likely reason.
In order to achieve high purity it is mandatory to minimize the amount of reboil that bypasses both stripping sections. Of course, the vapor to the refrigeration expander necessarily bypasses the strippers, so little margin is left for other bypass vapor. For crude argon to be withdrawn as product from the LP column overhead, it is very desirable that it be at least about 50% purity, and preferably better than 80% purity. Otherwise, so much product oxygen is lost with the crude argon that the recovery suffers, thereby negating the energy advantage. In order to achieve high enough purity in the overhead vapor of the low pressure column that crude argon can be withdrawn in preference to recycling, a relatively low LP column reflux temperature is required (corresponding to the higer argon content).
However, that reflux temperature gives rise to a correspondingly cold temperature for the vapor that is boiled thereby to become intermediate reboil for the MP column. Colder reboil means that it must be introduced at a higher intermediate location in the MP column. This requires that there by greater reboil in the MP column below that location either from the bottom reboiler (supplied by partially condensing air) or from an intermediate reboiler (suppled by HP column overhead). It is desirable to minimize both of the latter reboils. If there is greater (too much) reboil at the bottom of the MP column, the partially condensed air condensate will have greater N.sub.2 content, which requires a higher pressure for the same reboil temperature, and which also decreases the LN.sub.2 available from HP rectifier overhead, thus decreasing liquid reflux to MP overhead, thereby increasing O.sub.2 content in the nitrogen waste gas and thereby decreasing O.sub.2 product recovery. On the other hand, if there is greater reboil input to the MP intermediate reboiler from the HP rectifier overhead, that is also undesirable, because that reboil bypasses both stripping sections. This makes it harder or impossible to produce the desired oxygen purity--at the very least more stripping stages are required, which raises column pressure drops and hence required supply air pressure.
In summary, given the equipment configuration and process steps disclosed in the prior art disclosure, changes in operating conditions necessary to increase crude argon purity sufficiently to allow efficient withdrawal would be expected to cause completely offsetting and disadvantageous results in oxygen purity and recovery, and on the other hand the inefficient withdrawal of the low purity crude argon would cause a similarly disadvantageous decrease in oxygen recovery, and hence there was no preferential alternative to the disclosed crude argon recycle.
What is needed in order to efficiently produce high purity oxygen at high recovery plus crude argon byproduct, all at low energy input (low supply air pressure), is an efficient air reboiled triple pressure configuration which allows withdrawal of relatively pure (better than 80%) crude argon without the offsetting disadvantages described above. This is one major objective of the improvement disclosed below.
It is known that in distillation it is desirable to add heat (reboil) to the stripping (bottom) section of a distillation column over a range of tray heights or temperatures, and similarly for the rectifying (top) section to reject heat (i.e., add reflux) over a range of tray heights or temperatures. Several of the prior art idsclosures referred to above incorporate two or more discrete exchanges of heat from the HP rectifier to the stripping section of a lower pressure column. However, it is also known to conduct this heat exchange continuously over a range of tray heights. This is accomplished by "differential" or "non-adiabatic" distillation, as described in U.S. Pat. Nos. 3,508,412 and 3,563,047, 3,756,035, among others.
"Latent heat exchange" refers to an indirect heat exchange process wherein a gas condenses on one side of the heat exchanger and a liquid evaporates on the other, e.g. as occurs in the conventional reboiler/reflux condenser. Normally part of the heat exchange will also unavoidably be due to some sensible heat change of the fluids undergoing heat exchange--thus the label merely signifies the major mechanism of heat exchange, and is not intended to exclude presence of others.
"Air reboiling" is a latent heat exchange between partially condensing air and boiling distillation column bottom product, e.g. the MP column. Reboiling with partially condensing air as opposed to totally condensing air results in a more efficient configuration--the higher O.sub.2 content of the condensate allows a lower air pressure to be used to achieve a given reboil temperature.
Additional background art pertinent to this disclosure can be found in U.S. application Ser. No. 501,264 filed 6/6/83 by Donald C. Erickson, which is incorporated by reference.